The document provides a detailed description of the structure and functions of the lens in the eye, including its biochemistry, shapes, dimensions, and components such as the lens capsule, anterior lens epithelium, and lens fibers. It discusses the lens's growth and changes with age, including weight, refractive power, and protein composition, emphasizing the importance of crystallins for maintaining transparency. The lens is characterized as a crystalline, avascular, and nerve-free structure, intricately connected to the ciliary body by zonules.

1. Lens
Unit 3
Structure and Functions of Lens. Zonules.Biochemistry, Protein
fractions, Electrolytes, Dehydration and Transparency

2. Shape, transparency and Dimensions
• Lens is a transparent, biconvex, crystalline
structure placed between iris and the vitreous in
a saucer-shaped depression, the patellar fossa.
• To be more precise, the lens is an asymmetric
oblate spheroid
• The posterior surface of the lens capsule is in
intimate contact with the vitreous in this fossa
and is attached to it in a circular area with
ligamentum hyaloid capsular (Wiegert's ligament)
• Inside this circle, between hyaloid face and the
lens capsule is a small cavity or potential space
called as retrolental or Berger's space.

3. Shape, transparency and Dimensions
• Equatorial diameter of the lens, about 6.5 mm at birth, increases to 9-10 mm in the second decade and then
remains almost constant
• Thickness (axial or anteroposterior diameter) varies with age between 3.5 mm (at birth) and 5 mm (at extreme
of age)

4. Shape, transparency and Dimensions
• Weight of lens varies with age
• At birth, the weight is about 65 mg, which increases rapidly to about 125 mg by the end of first year
• It then increases approximately 2.8 mg/year until the end of first decade reaching about 150 mg
• Thereafter the rate of increase slows to reach a weight of approximately 260 mg at 70-80 years of age
• In males, the lens weighs more than the aged matched females, with a mean difference of 7.9 +2.47 mg.
At birth
65 mg
First year
125 mg
10th year
150 mg
70th - 80th year
260 mg
2.8 mg/year Slow rate

5. • Surfaces: It has two surfaces. The anterior surface, less convex than the posterior, is the segment of a sphere
whose radius averages 10 mm (8-14 mm)
• The posterior surface, more curved than the anterior, presents a radius of about 6 mm (4.5-7.5 mm)
• Equator: These two surfaces of lens meet at the equator, which is almost circular and has a rippled or undulated
appearance
Shape, transparency and Dimensions

6. Poles
• The centre of the anterior and posterior surface is called as the anterior pole and posterior pole,
respectively
• The anterior pole is about 3 mm from the back of cornea
• Refractive index of the lens is 1.39 (nucleus 1.42, cortex 1.38)
• Its refractive power is about 16 -17 dioptres
Shape, transparency and Dimensions

7. • Accommodative power varies with age, being 14-16 D at birth; 7-8 D at 25 years of age and 1-2 D at 50
years of age
• Colour of the lens also changes with age. A transparent lens in infants and young adults is colourless,
acquires a definite yellow tinge after about 30 years of age and appears amber coloured in old age.
• Consistency of the lens cortex differs from the nucleus; the former being softer than the latter.
Shape, transparency and Dimensions

8. Lens is a crystalline structure that is avascular and is devoid of nerves and connective tissue
It consists of three distinct part:
1. Lens capsule
2. Anterior lens epithelium, and
3. Lens substance or lens fibres
Structure of lens

9. • Lens capsule is a thin, transparent, hyaline collagenous membrane which surrounds the lens completely
• capsule is highly elastic but does not contain any elastic tissue
• Capsule is secreted by the basal cell area of the lens epithelium anteriorly and by the basal area of the
elongated fibres posteriorly
• Produced continuously through life, the lens capsule is the thickest base in membrane in the body
Structure of lens: Lens capsule

10. • Capsule thickness varies according to the age and is not consistent through its extent
• It is thicker anteriorly than posteriorly and at the equator than the poles, being thinnest at posterior
pole
• On light microscopy, the capsule appears as a homogenous structure
• on ultramicroscopic examination, it shows a lamellar appearance.
• Each lamella contains fine filaments
Structure of lens: Lens capsule

11. • In true exfoliation of the lens capsule, superficial zonular lamella
of the capsule splits off from the deeper layer
• Lens capsule is composed principally of type IV collagen and 10%
glycosaminoglycans
• It contains enzyme, ATP and glycolytic intermediates but cannot
be considered to have an independent metabolism
• There are chemical and antigenic similarities between the lens
capsule and basement
Structure of lens: Lens capsule

12. • single layer of cuboidal nucleated epithelial cells and lies deep to the anterior capsule extending upto
equatorial lens bow
• These cells contain all the organelles found in a typical epithelial cell
• Lens epithelial cells contain three cytoskeletal elements, microfilaments (actin), intermediate filaments
(vimentin) and microtubules (tubulin)
Structure of lens: Anterior lens epithelium

13. • Almost all the metabolic, synthetic and transport processes of the lens occur in this layer
• In the equatorial region, these cells become columnar, are actively dividing and elongating to form new
lens fibres throughout life
• There is no posterior epithelium, as these cells are used up in filling the central cavity of the lens vesicle
during development of the lens
Structure of lens: Anterior lens epithelium

14. The anterior lens epithelium can be divided into three zones:
1. Central zone
• It consists of cuboidal cells which are polygonal in flat section
• These cells are approximately 10 um high and 15 um wide
• Their nuclei are round and located slightly apically. These cells are
stable and their number slowly reduces with age
• Under normal circumstances, these cells do not mitose, but can do
so in response to a wide variety of injurious insults including uveitis
• During injury repair, epithelial cells are elongated; resembling
fibroblasts and can pile up to 10 layers thick under the capsule.
Structure of lens: Zones of lens epithelium

15. The anterior lens epithelium can be divided into three zones:
2. Intermediate zone
• It consists of comparetively smaller and more cylindrical cells located
peripheral to the central zone
• Their nuclei are round and central
• These cells mitose occasionally
Structure of lens: Zones of lens epithelium

16. The anterior lens epithelium can be divided into three zones:
3. Germinative zone
• It consists of columnar cells which are most peripheral and located
just pre equatorial
• Nuclei of these cells are flattened and lie in the plane of cell axis
• Cells of the germinative zone are actively dividing to form new cells
which migrate posteriorly to become lens fibres
• This process continues throughout life
• These cells are extremely susceptible to irradiation
Structure of lens: Zones of lens epithelium

17. Structure of lens: Zones of lens epithelium

18. Formation
• The epithelial cells elongate to form the lens fibres
• At first, the lens fibres are formed from the posterior epithelium which runs from posterior to anterior to fill
the lens vesicle
• But later on, the lens fibres are derived from the cells of the equatorial region of the anterior epithelium
Structure of lens: Lens substance or lens fibers

19. • These cells divide, elongate and differentiate to
produce long, thin, regularly arranged lens fibres that
constitute the bulk of the lens
• Successively, the new lens fibres are laid on the older
deeper fibres
• The superficial (new) fibres are nucleated with
elongation of the cell; the nuclei assume a relatively
more anterior position
• As the new fibres are laid down, the anterior shifted
nucleus forms a line convex forward at the equator,
known as lens or nuclear bow
Structure of lens: Lens substance or lens fibers

20. Structure of lens fibres
• On cross-section, the lens fibres are almost hexagonal in shape and are bound together by the ground
substance
• The cytoplasm of the cells of the superficial bow region and the newly formed lens fibres contain a
nucleus, mitochondria, Golgi apparatus, rough endoplasmic reticulum, and polysomes
Structure of lens: Lens substance or lens fibers

21. • The ribosomal content of the newly formed lens fibres is more
than the epithelial cells indicating an elevated protein synthesis
• The nuclei of the lens fibres are present temporarily and
disappear later on
• Thus the cytoplasm of the older lens fibres is devoid of nuclei, is
homogenous and granular with very few organelles
• There are interlocking processes between cells (ball-and-socket
and tongue-and groove interdigitations) with zonulae
occludentes present
• Both desmosomes and tight junctions are absent from the
mature lens fibres, although desmosomes are found between
elongating fibres
Structure of lens: Lens substance or lens fibers

22. • It is interesting to note that the interdigitations are less
complicated in the superficial zone of the lens; and this may
permit moulding of the lens shape during accommodation
Structure of lens: Lens substance or lens fibers

23. Structural arrangement of the lens fibres
• The initial fibres forming the fetal nucleus just surrounding the
embryonic nucleus are arranged in such a way that they
terminate with two Y-shaped suture on the anterior (upright Y)
and the posterior (inverted Y) surface of the lens
Structure of lens: Lens substance or lens fibers

24. Structural arrangement of the lens fibres
Structure of lens: Lens substance or lens fibers
• Later in gestation and following birth, the growth of
the lens sutures is much more irregular
• Instead of simple Y-sutures, more complicated
dendritic patterns are observed due to asymmetrical
fibre growth
• The formation of sutures enables the shape of the lens
to change from spherical to a flattend biconvex sphere
A) Embryonic
B) Adult lens
fibre
arrangement

25. Zonal arrangement of the lens fibres
Structure of lens: Lens substance or lens fibers
• The lens fibres are formed throughout life and are
arranged in zones that delineate the various periods of
development of the lens
• This stratification is due to optical differences between
the older, more sclerotic regions of the central lens
and the newer, more transparent peripheral areas
• In an adult, the lens fibres are arranged compactly as
nucleus and cortex of the lens

26. Zonal arrangement of the lens fibres
Structure of lens: Lens substance or lens fibers
A. Nucleus.
• It is the central part containing the oldest fibres. It
consists of different zones
• Embryonic nucleus is its innermost part (formed at 1
to 3 months of gestation)
• The embryonic nucleus contains the original primary
lens fibre cells that are formed in the lens vesicle

27. Zonal arrangement of the lens fibres
Structure of lens: Lens substance or lens fibers
• Outside the embryonic nucleus, successive nuclear
zones of secondary lens fibres are laid down
concentrically, encircling the previously formed
nucleus as the development proceeds, and depending
upon the period of formation, are called:
• Fetal nucleus (corresponding to lens from 3 months of
gestation till birth)
• Infantile nucleus (corresponding to lens from birth to
puberty) and
• Adult nucleus (corresponding to the lens in adult life)
Note. The size of the embryonic and fetal nuclei remains
constant while that of adult nucleus is always increasing

28. Zonal arrangement of the lens fibres
Structure of lens: Lens substance or lens fibers

29. The ciliary zonules
• The ciliary zonules (zonules of Zinn or
suspensory ligament of lens) consist essentially
of a series of fibres which run from the ciliary
body and fuse into the outer layer of the lens
capsule around the equatorial zone
• Thus, they hold the lens in position and enable
the ciliary muscle to act on it

30. The ciliary zonules: Structure
• The zonular fibres are transparent, stiff and not elastic. Each zonular fibre has a diameter of about 0.35 to
1.0 micro meter
• It is composed of microfibrils with a diameter varying from 8-40 nm
• Zonular fibres are composed of glycoproteins and mucopolysaccharides and are similar in structure to the
microfibrils of the elastic fibres
• Their susceptibility to hydrolysis by a chymotrypsin has been used to advantage in intracapsular cataract
surgery
• Structurally, three different types of zonular fibres have been described
1. First type fibres. These are thick, about 1 m in diameter, wavy and usually lie near the vitreous
2. Second type fibres. These are thin and flat.
3. Third type fibres, These are very fine and run a circular course.

31. The ciliary zonules: Gross appearance

32. Arrangement of Zonular Fibres
A. Main fibres of the ciliary zonules
• The main fibres of the ciliary armies which bind the lens
with the ciliary body, depending upon their arrangement
can be classified into the following four groups
1. Orbicular Posterior capsular fibres
• These are the most posterior and innermost zonular fibres
• These take origin from the ora serrata, pass anteriorly in
close contact with the anterior limiting layer of the
vitreous and are inserted together with hyaloid capsular
ligament in the posterior capsule of the lens
• Structurally, they are second type fibres.

33. Arrangement of Zonular Fibres
2. Orbicular Anterior capsular fibres
• These are the thickest and strongest (structurally of first type)
zonular fibres
• They arise from the pars plana of ciliary body (orbicularis
ciliaris), pass anteriorly to get inserted anterior to the equator
• By the supporting (auxiliary) fibres, they are attached to the
valleys and sides of the ciliary processes
3. Cillioposterior capsular fibres
• These are the most numerous zonular fibres
• They arise mainly from the valleys and a few from the sides of
the ciliary processes, pass posteriorly and get inserted on the
posterior capsule anterior to the insertion of the orbiculo-
posterior capsular fibres.

34. Arrangement of Zonular Fibres
4. Cillioequatorial fibres
• These fibres arise from the valleys of the ciliary processes
and pass almost directly inward to be inserted at the equator
• They occupy the whole of the interval between the anterior
and posterior group of fibres
• These are third type of fibres
• These are present in abundance in youthful eyes and tend to
disappear and become sparse with advancing age
B. Auxiliary fibres
• The auxiliary or supporting fibres provide strength to main
fibres by anchoring the individual portions of zonules
• These also help to hold the various portions of the ciliary
body together

35. 34% Protein65% Water
Biochemical composition of lens
Water constitutes about
65% of the lens wet
weight
Of the solids, the highest is
protein which constitutes about
34% of the total weight of an
adult lens
lipids, inorganic ions,
carbohydrates, ascorbic
acid, glutathione and
amino acids

36. }65% Water
Biochemical composition of lens: Lens water
• Lens is a relatively dehydrated organ, cortex being more hydrated than nucleus
• maintained by an active sodium pump that resides within the membrane of the cell, in the lens epithelium and
in each lens fibre
• Fischer suggested that out of this about 80% is free while remaining is bound water
• A small portion of the lens water is located in the extracellular space
• Low amount of water in the lens is a natural consequence of the need for having a refractive index quite
different from that of the watery fluids at the two optical interfaces of the lens
• The normal human lens does not show significant alteration in hydration with age
80% free water

37. Proteins
Soluble,
Albumin
Insoluble,
Crystallin
Biochemical composition of lens: Proteins
• Protein content of the lens is higher than that of any other organ in the body
• The physical state of proteins is an important factor for the maintenance of transparency
• Morner for the first time classically divided the proteins of crystalline lens of cattle into
an insoluble fraction called albumin and the soluble fraction called crystallins
• The soluble fraction has three components namely alpha-, beta- and gamma crystallin
• The three crystallins can be separated by precipitation at different pH, by salting out, by
electrophoresis or by running through cellulose column

38. Biochemical composition of lens: Proteins
Krause studied the various protein fractions in the lens as follows:
Proteins Insoluble albumin 12.5%
Alpha – Crystallin 31.7%
Beta – Crystallin 53.4%
Gamma – Crystallin or albumin
1.5%
Mucoproteins
0.8 %
Nucleoproteins
0.07%
Soluble
fraction

39. Biochemical composition of lens: Proteins
Proteins
• few minor proteins reported in the lens are glycoprotein, phosphoprotein, lipoprotein and fluorescent
proteins
• cortex contains more soluble proteins than nucleus which contains more insoluble proteins
• The cortex of the young lens practically contains no albuminoid, whereas the nucleus of old lens is composed
of almost entirely of albuminoid
• alpha-crystallin and albuminoid are immunologically similar
Alpha Crystallin
(Soluble)
Albuminoid protein
(Insoluble)
Age ↑

40. Biochemical composition of lens: Proteins
Soluble proteins
• The lens crystallins make up the bulk of refractive fibres of the lens and are considered structural proteins
• synthesis of soluble proteins takes place in the equatorial part and on the surface of lens
• The newly formed fibres contain very little or no albuminoid
• A part of the soluble lens proteins may be formed in deeper lens fibres, at least in those which contain nuclei
• The high concentration of crystallins and the gradient of refractive index are responsible for the refractive
properties of the lens
• The short range order of these proteins ensures that the lens remains transparent

41. Biochemical composition of lens
Alpha-crystallin
• the highest molecular weight and at alkaline pH it has the greatest positive
charge
• Average molecular wt of about 106
• composed of several polypeptides held together by non-covalent forces
• The molecular wt of the A chains is 19,500 and of the B chains 22,500
• The A chains contain one thiol group per chain while B chains appear to
contain no thiol group
• alpha-crystallins are polymers and consist approximately of 50 monomers
• They are the largest crystallin

42. Biochemical composition of lens
Beta-crystallins.
• This fraction of lens proteins is a heterogenous group of proteins with molecular
wt from 5 x 104 to 2 x 105
• The beta-crystallins contain many polypeptide chains, some of which appear to be
present in aggregates
• Shapiro has reported that this protein group contains three different polypeptide
chains of the molecular weights of 21000, 23000 and 29000
• They also have a relatively high thiol content and might have disulphide linkages

43. Biochemical composition of lens
Gamma-crystallin
• These are composed of monomers only
• This fraction constitutes about 60% of soluble lens proteins in young rat and dog
fish
• As the lens ages, there is a progressive and significant decrease in the relative
concentration
• Gamma-crystallin level is high in nucleus and low in cortex, especially in young
cortex
• These are the smallest crystallins.

44. Biochemical composition of lens
Insoluble proteins
• The chief insoluble protein is albuminoid - 12.5% of total protein
• Molecular wt - 3,70,000
• Found as a mixture - it is only partly digested by urea - extracted by soidic aqueous solution
• The amino acid composition of this protein is similar to alpha crystallin
• In human lens, most of the albuminoid is urea Soluble and appears to be derived from the alpha-crystallins

45. Biochemical composition of lens
Other lens proteins
• Glycoproteins are a group of proteins to which sugars are covalently bound
• They are primarily associated with the lens cell membrane and are of considerable importance
• They also contribute to the intercellular ground substance.
• The lens cortex contains considerably more glycoproteins than the nucleus
• Some other rare lens proteins include nucleoproteins, phosphoproteins, lipoproteins and fluorescent
proteins

46. Biochemical composition of lens: Amino acids
Amino Acids
Proteinogenic
alanine, leucine, glutamic acid, aspartic acid, glycine, valine,
phenylalanine, tyrosine, serine, isoleucine, lysine, histidine,
methionine, proline, threonine and arginine
Non - proteinogenic
taurine, alpha-amino butyric acid, ornithine, 1-methyl-histidine,
3-methyl-histidine and homo-carnosine
• lens contains all the amino acids present in any other tissue except tryptophan, cysteine and possibly
hydroxy-proline
• Concentration of each amino acid is higher in the lens as compared to aqueous humour vitreous humour
leading to a conclusion that they are actively transported into the lens

47. Biochemical composition of lens: Amino acids
• An active process is necessary to ensure that protein synthesis is not limited by the availability of amino
acids
• the free amino acid pool is quite characteristic and constant for similar animals
• amino acid concentration of lens is not appreciably affected by ageing, fasting or feeding a protein-free diet
• unaltered level of amino acids in lens may be a result of balance of protein synthesis and catabolism on one
hand and amino acid excretion, uptake by lens and the synthesis and breakdown of amino acids

48. Biochemical composition of lens: Carbohydrates
Carbohydrates
Free
glucose, fructose, and glycogen
Derivatives sorbitol, inositol, ascorbic
acid, gluconic acid and glucosamine

49. Biochemical composition of lens: Carbohydrates
Glucose
• The glucose level - vary from 20-120 mg%
• reported still lower concentration of glucose in
lens in various species
• The lenticular glucose has its source in aqueous
humour
• The level of glucose in lens is 1/10th of
aqueous, where glucose concentration has
been found to be 100 mg%.
Fructose
• It is produced from glucose in the crystalline
lens
• Concentration of fructose varies considerably in
many species
• Its concentration also varies with age
• 6 mg - 14th day & 50 mg-100 mg at 550th day
(Rats)

50. Biochemical composition of lens: Carbohydrates
Glycogen
• Only traces of glycogen have been found in
mammalian lenses
• Concentration varies with age and the region of lens
• localised principally in the nucleus where it appears
to replace gamma-crystallin
• normally present there to functionally to increase
refractive index
• It is located as a thin layer of intracellular granules,
surrounding the nuclei of epithelial cells
Sorbitol
• Found in normal lens of several species
• 17 mg/100 g fresh lens of rabbit
Inositol
• found in the lens of several species
• Its function is unknown although it may possibly be
involved in the metabolism of phospholipids

51. Biochemical composition of lens: Lipids
• 2.5% of wet weight
• cholesterol, various phospholipids such as cephalexin, isolecithin, sphingomyelin and glycerides
• human lens had lipids in two forms, a free form and a bound form as lipoproteins
• The proteolipids constitute 2% of the wet weight of lens and that 65% of lenticular lipids are bound to
proteins
• lipids are most abundant in epithelial cells in children and in the cortex in adults
• it may function as a lubricating cement substance
• lipid content (cholesterol) increases with age especially in nucleus while the glycerides decrease
• Similar changes occur in cataract where lecithin is abundant and cholesterol is found as crystals
• In cataracts the concentration of free lipids increases and lipoprotein decreases.

52. Biochemical composition of lens: Electrolytes
Potassium
• It is the predominant cation in lens
• Its concentration in fresh human lens has been
reported to vary between 114 and 130 mEq/kg lens
matter
• The levels are higher than in any other eye tissue
• This high level is probably a result of the unusually
large proportion of intracellular space in lens
Sodium
• Its concentration in lens is about 10 to 50% of the
potassium
• In human lens, the sodium concentration is about 14-
25 mEq/kg lens water
• There is some variation in these levels between
species and a marked regional variation in the
concentration of sodium, which is more than twice in
the superficial cortex as compared to central nucleus.

53. Biochemical composition of lens: Electrolytes
Calcium
• normal young lens has one of the lowest of
all tissue calcium levels
• A mean value of 0.14 mg/mg dry weight is
reported for human lenses.
Anions
• The main anions of the lens are chloride,
bicarbonate, phosphate and sulphates
• Phosphate is the predominant anion in the
lens, comprising nearly half the ash
• The level reported for the total phosphate is
240 mg/ 100 g and for inorganic phosphate
25 mg/100 g in young calf lens

54. Biochemical composition of lens: Glutathione
• The content of glutathione in the lens depending on the species and the method used for its estimation
has been reported to vary from 3.5 to 5.5 mm/g wet weight of the lens
• The level of glutathione in the lens is also known to be altered with the age of the individual
• Its concentration falls with advancing age
• This decrease in the level of glutathione with advancing age is relative and not absolute
• It is because of the increase in the wet weight of the lens with age
• The lens is constantly exposed to attack by oxidative agents; indeed there is a high level of hydrogen
peroxide in normal aqueous and peroxidase activity is also present in the lens itself

55. Biochemical composition of lens: Glutathione
• Several enzyme systems are available to minimize or buffer the effects of oxidants, including catalase,
superoxide dismutase, glutathione peroxidase and glutathione-S-transferase
• The lens contains high levels of glutathione with the highest concentration in the epithelium
• Catalase and low levels of superoxide dismutase have also been identified in lens epithelium concluding
that these systems are also probably important
• Glutathione is also important in protecting thiol groups in proteins, especially cation transporting
membrane proteins in the lens, which additionally accounts for its unusual high concentration in this
tissue
• More than 95% of glutathione is in the reduced state

56. Biochemical composition of lens: Ascorbic Acid
• Mean value of ascorbic acid in an adult man is 30 mg/100 gm of the wet weight of the lens
• In the aqueous humour, ascorbic acid is actively transported to a concentration some 15 times greater
than that in plasma
Though the ascorbic acid content of the lens is even greater than that of aqueous humour, it is neither
synthesized nor actively transported into the lens
• Its accumulation within the lens might be explained by assuming that a portion of ascorbic acid is protein
bound
• the conversion between ascorbic acid and the oxidized form, dehydroascorbic acid, might be coupled with
other oxidation reduction systems in the lens